I guess with the 28 man roster, and 14 or 15 man staffs, you might be able to work some sort of "piggyback" schedule. But with a 13 man staff-already limiting yourself to 13 position players-it's really difficult to tie up 7 spots to starters. And that's what you're doing...7 spots for guys that only pitch every 5th day. That leaves you 6 relievers, to cover at minimum something like 11 innings those every 5 days. And that's if everything goes perfectly...you get 6 IP from the regular starters every day, and 8 from each of the 2 piggyback days. No extra innings games. You can save an inning on the road in losses, but in reality, we all know it won't work perfectly. You'll be asking for way more than 11 IP every five days from those 6 relievers. Way more.
And that ignores the simple truth...do we HAVE 7 guys capable of regular 4 or 5 inning stints? I'd question whether we have 5, much less 7.
There's just no way that works. I think it's near impossible, which is why nobody is doing it.